Loop antenna

ABSTRACT

In a directional loop antenna, particularly for television receivers, in which a pair of arcuate conductive members are mounted in opposed relation with their concave sides facing each other to define a loop, a dummy load is connected between two of the adjacent ends of the conductive members and output terminals are connected to the opposite adjacent ends, the conductive members are formed with relatively narrow end portions and relatively wide middle portions to increase the frequency band width and gain of the antenna.

United States Patent Inventors Appl. No. Filed Patented AssigneePriority Motomu Tadama Kanagawa-ken;

Kosuke Akiba, Tokyo; Toshitada Doi, Kanagawa-ken; Masashi Mikkaichi;Tetsuya Mori; Risaburo Sato, Miyagi-ken, Japan Feb. 7, 1969 Apr. 6 l 971 Sony Corporation Tokyo, Japan Feb. 8, 1968 Japan LOOP ANTENNA 1 Claim,20 Drawing Figs.

US. Cl

Int. Cl Field of Search.....

[56] References Cited UNITED STATES PATENTS 2,116,734 5/1938 Reinartz343/882X 2,247,743 7/1941 Beverage... 343/732 2,501,430 3/1950 Alford343/741 2,551,664 5/1951 Galper 343/741 FOREIGN PATENTS 973,146 12/1959Germany 343/744 Primary Examiner-Herman Karl Saalbach AssistantExaminerMarvin Nussbaum AttorneysAlbert C. Johnston, Robert E. lsner,Lewis H.

Eslinger and Alvin Sinderbrand I Patented April 6, 1971 4 SheetsSheet 1INVENTORS MOTOMU TADAMA KOSUKE AKIBA TOSHITADA DOI MASASHI MIKKAICHITETSUYA MORI RISABURO SATO CUT-OFF FREQUF/VCY BAND W/DTH F I G. 4.

BY L

ATTORNEY Patnted April 6, 1971 3,573,830

4 Sheets-Sheet 4 L FIG. 15.

W'(c=4g) K INVENTORS h f ao) MOTOMU TADAMA KOSUKE AKIBA TOSHITADA DO!MASASHI MIKKAICHI TETSUYA MORI RISABURO SATO I I I BY. 50 100 750 200250 M ---f (MC/5) A TTORNE Y LOOP ANTENNA This invention relatesgenerally to directional antennas suitable for use as the antennas oftelevision receivers, and more particularly is directed to improvementsin directional loop antennas.

If a four-terminal circuit is considered as an antenna, the imageimpedance of such circuit consists of an imaginary part indicating thatthe antenna has a wide frequency band in which a sharply decreased gainis encountered and, therefore, there is a deterioration in thedirectivity ofthe antenna and also an extremely reduced impedance for afairly wide range of frequencies so that such four-terminal circuit isnot usable as an actual antenna. In the following description, thefrequency bandwidth within which the sharply decreased gain of anantenna is encountered will be referred to as the cutoff frequencybandwidth." If the image impedance of the antenna consists of a realpart and an imaginary part, the cutoff frequency band width will extendover those frequencies at which the imaginary part of the imageimpedance is greater than the real part thereof.

It has been proposed to provide a loop antenna which comprises a pair ofsubstantially semicircular, elongated metal plates of uniform widthmounted in opposing relation with their ends adjacent each other todefine a loop having a diameter very substantially less than thetransmitted wavelength, for example, one fifth of such wavelength, inorder to obtain a directional characteristic of the cardioid type. Insuch previously proposed loop antenna, the cutoff frequency bandwidthis, nevertheless undesirably wide.

Accordingly, it is an object of this invention to provide a loopantenna, particularly suited for use with television receivers, and inwhich the cutoff frequency bandwidth is substantially reduced so as tohave a wide frequency band and high gain.

Another object is to provide a loop antenna of small size having theforegoing characteristics;

A further object is to provide a loop antenna, as aforesaid, having aunidirectional characteristic and avoids the reception of ghost images.

In accordance with an aspect of this invention, a loop antenna isprovided in which a pair of arcuate, preferably substantiallysemicircular conductive members are mounted in pposed relation withtheir concave sides facing each other to define a loop, a dummy load isconnected between two of the adjacent ends of the conductive members,output terminals are connected to the other adjacent ends of theconductive members, and the conductive members have relatively narrow,preferably tapering end portions and relatively wide middle portions sothat the characteristic impedance of the antenna is substantiallyuniform, whereby to reduce the cutoff frequency band width and thusincrease the frequency bandwidth and gain of the antenna.

The above, and other objects, features and advantages of the invention,will be apparent in the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings, wherein:

FIG. 1 is a diagrammatic view illustrating a loop antenna as previouslyproposed;

FIG. 2 is a view similar to that of FIG. 1, but showing a firstembodiment according to this invention;

FIG. 3 is a diagrammatic perspective view showing another embodiment ofthis invention;

FIG. 4 is a graph showing the relation between the cutoff frequencybandwidth of a loop antenna as shown in FIG. 3 and the length of thetapered ends of its conductive members;

FIG. 5 is a diagrammatic view showing shapes of conductive members forloop antennas according to this invention, as developed on a flat plane;

FIG 6 is a diagrammatic perspective view referred to explaining thecurrent distribution in an arcuate conductive member;

FIGS. 7 and 8 are diagrammatic perspective views showing the conductivemembers for two further embodiments of this invention;

FIGS. 9A and 9B are graphs respectively showing the reactance andresistance components of the dipole and loop impedances of an antenna ofthe type shown on FIG. 7;

FIG. 10 is a graph showing the image impedance characteristic of theloop antenna shown on FIG. 7;

FIGS. 11, 12 and 13 are graphs similar to FIGS. 9A, 9B and 10,respectively, for an antenna similar to that of FIG. 7, but in which,contrary to this invention, the width of each conductive member does notdecrease from a maximum at the middle toward minimums at the ends;

FIG. 14 is a graph similar to that of FIG. 10, but for a loop antennaaccording to the embodiment of this invention shown on FIG. 8;

FIGS. 15 and 16 are perspective views showing a loop antenna accordingto this invention particularly adapted for use with portable televisionreceivers and which is shown in two different positions; and

FIGS. l7, l8 and 19 are schematic views illustrating the directionalcharacteristics of the antenna of FIG. 7 for waves arriving in threedifferent directions in relation to the antenna.

Referring to the drawings in detail, and initially to FIG. 1, it will beseen that a previously proposed loop antenna, as there illustrated,comprises a pair of substantially semicircular conductive members 1 and2 arranged in opposing relation with their concave sides facing eachother, a dummy load 3 connected between two of the adjacent ends ofmembers 1 and 2, and output terminals 4 connected to the other adjacentends of the conductive members. The conductive members 1 and 2 of suchknown loop antenna are of uniform width along their lengths both in thedirection of their curvature and in directions perpendicular thereto. Inthe case of a loop antenna of the above-described type, the cutofffrequency bandwidth is influenced by the following factors:

1. The types of wave modes, other than the TEM mode, ar-

riving at the antenna;

2. The radiation load on the antenna; and

3. Variations in the characteristic impedance of the anten- With respectto factor (1) above influencing the cutoff frequency bandwidth, it willbe seen that the various types of wave modes result in a discrepancybetween the direction of the currents flowing in the antenna and thepropagating directions of the waves arriving at the antenna. One ofthese waves is a horizontally polarized wave. When the antenna of FIG. 1receives a horizontally polarized wave propagated in the plane ofarcuate conductive members 1 and 2, for example, as indicated by thearrow A, electrical inductive currents i and 11. and a magneticinductive current i are generated in conductive members 1 and 2 by amagnetic field H and an electrical field E, respectively, of the wave. Afigure-eight directive characteristic is obtained from the electricalinductive currents i and and a circular characteristic'is obtained fromthe magnetic inductive current i and such figure-eight and circularcharacteristics combine to provide the desired cardioid characteristic.Thus, although the cutoff frequency bandwidth could be narrowed bysuppressing the horizontally polarized wave, this would undesirablyremove the cardioid characteristic of the antenna.

The radiation load mentioned in (2) above, is caused by an increase inthe resistance component of the antenna impedance which, in turn,depends upon wave frequency. Although the cutoff frequency bandwidth canbe narrowed by decreasing the radiation load, this is achieved at theexpense of a deterioration of the antenna function.

With respect to factor (3) mentioned above as influencing the cutofffrequency bandwidth, it should be noted that, in the case of a pair ofparallel conductive members, the characteristic impedance W is expressedas follows:

W276 log T (I) where 2p is the lateral distance between the conductivemembers, and Zr is the width of each member. Hence, for the loop antennashown on FIG. 1, where the lateral distance 2p between members 1 and 2varies from one end to the other end thereof, and where the width 2r ofeach of members 1 and 2 is constant, the characteristic impedance W ofthe loop antenna varies and thereby contributes to a relatively widecutoff frequency bandwidth for the antenna.

Thus, in accordance with the present invention, the cutoff frequencybandwidth of a loop antenna is narrowed by providing the same with asubstantially uniform characteristic impedance, that is, by influencingthe above-mentioned factor (3). As is apparent from equation (I) above,the substantially uniform characteristic impedance is convenientlyachieved by varying the width of each of the conductive members inaccordance with the varying distance between the conductive members.More specifically, in the loop antenna having arcuate conductive membersarranged in opposing relation with their concave sides facing each otherso that the maximum distance between the conductive members is at themiddle of the latter and such distance decreases progressively from themiddle of the conductive members to the ends thereof, a substantiallyuniform characteristic impedance is achieved by providing the conductivemembers with relatively wide middle portions and with relativelynarrowed end portions.

Referring now to FIG. 2, it will be seen that, in an embodiment of thisinvention as there shown, the arcuate conductive members 1a and 2a haverelatively wide middle portions and relatively narrow end portions, withthe dummy load 3a being connected between two of the adjacent ends ofmembers 1a and 2a and the output terminals 4a being connected to theother adjacent ends of the conductive members. In the embodiment of FIG.2, the variations in the width of each of members In and 2a are indirections that are generally radial with respect to the curvaturethereof, that is, each conductive member may be formed of a conductiveplate lying in a flat plane and having arcuate inner and outer edgeswith different centers of curvature to establish the curvature of theplate in such flat plane and the width variations thereof. Theembodiment of FIG. 2, however, tends to reduce the area enclosed by theloop, and hence tends to reduce the characteristic of the antenna.

Accordingly, as shown on FIG. 3, it is preferred that the widths ofconductive members 1b and 2b be varied between their middle and endportions in directions that are perpendicular to the direction ofcurvature thereof. Thus, in the embodiment of FIG. 3, the conductivemembers 1b and 2b may be formed of metal plates that are arcuately bowedout of flat planes so as to be substantially of semicircular shape andthat have varying widths, in directions parallel to the axis ofcurvature, between maximum widths d at the middle portions of the curvedplates and minimum widths d at the ends of tapered end portions of theplates. As before, the dummy load 3b is connected between two of theadjacent ends of plates lb and 2b, and the output terminals 4b areconnected to the other two adjacent ends of the plates.

In a typical example of a loop antenna according to the embodiment ofthe invention shown on FIG. 3, the diameter across the loop, that is,the maximum diametrical distance between the middle portions ofconductive members 1b and 2b is 300 mm., the width d at the relativelywide middle portion of each conductive member is 114 mm., the width d'at the ends of each conductive member is mm., and the end portions ofeach plate or conductive member taper from the maximum width d to theminimum width d over a length b, for example, as shown on FIG. 5.

FIG. 4 illustrates the variations in the cutoff frequency bandwidth ofthe above dimensional example that result from changes in the value ofb, that is, in the length of the tapered end portions. On FIG. 4, thecurve f is for a first resonance frequency of a dipole impedance Z,,,that is, the impedance of the antenna with an infinite dummy load, andthe curve f is for a first antiresonance frequency of a loop impedanceZ, that is the impedance of the antenna with a zero dummy load. It willbe appreciated that for each value of b, the frequencies between curves1, and f constitute the cutoff frequency bandwidth of the antenna. Thus,if b is zero, that is, if conductive members 1b and 2b have the width d(1 14 mm.) uniformly along their entire lengths, the cutoff frequencybandwidth of the antenna extends from about Mc/s. to about Mc/s., whichrepresents a bandwidth of about 45 Mc/s. It will further be seen that,for values of b between 30 and 90 mm., the cutoff frequency bandwidth isreduced in accordance with this invention to about 20 Mc/s., and thatthe minimum cutoff frequency bandwidth is achieved when b has a value ofabout 60 mm.

The characteristic impedance of the loop antenna according to the abovedimensional example, and having the tapered end portions each extendingover the length b of 60 mm., is about 200 ohms. Of course, where theconductive members of the loop antenna according to this invention havemiddle portions with parallel edges and the width of each of suchconductive members varies only at end portions each extending over thelength b, for example, as indicated at 5a on FIG. 5 where the conductivemember is shown developed in a flat plane, the

characteristic impedance is not exactly uniform, but only approximatesthe uniform condition. Shapes for the conductive members that wouldresult in exactly uniform characteristic impedances are shown indot-dash lines and in dotted lines at 5b and 50, respectively. Inaccordance with the foregoing equation (I) it will be appreciated thatthe shapes 5b and 50 have edges in the form of sine curves. In order toobtain the minimum cutoff frequency bandwidth for a loop antenna havingits conductive members in accordance with the shape indicated at 5a onFIG. 5, it is desirable that the tapered end portions of each conductivemember extending over the length b have edges corresponding torespective portions of a sine curve, for example, corresponding torespective portions of the since curve defining the edges of shape Sb,as shown. By way of comparison, it may be noted that loop antennashaving conductive members with shapes as indicated at 5b and 51: willhave characteristic impedances of 200 and 300 ohms, respectively. Thus,the shape indicated at 5a achieves substantially the same characteristicimpedance as the shape indicated at 5b, but is formed from less materialand has a smaller maximum width d so as to minimize the overalldimensions of the loop antenna.

In the embodiments of the invention illustrated by FIGS. 2 and 3, theconductive members have been constituted by solid metal plates that aresuitably shaped. However, in the case of a curved plate P (FIG. 6)having an axial dimension B, the distribution of circumferentialcurrents i at various distances Z from the circumferential median of theplate P is given by the equation:

in which C is a constant. From equation (II) it will be apparent thatthe currents through the middle portion of plate P between the oppositecircumferential edges thereof (that is, whenZEO) are so small that themiddle portion of such plate can be omitted, or provided with anopening, without substantially altering the conditions for current flowthrough plate P.

Thus, in loop antennas according to this invention, each of the arcuateconductive members may have an opening extending therealong and being ofa shape that is similar to that of the perimeter of the conductivemember. For example, as shown on FIG. 7, the conductive members 10 and2c of a loop antenna according to this invention may be convenientlyformed of wirelike elements 11a and 11b and wirelike elements 12a and12b, respectively, which have circular or other cross sections and whichare joined to each other at the ends of the respective conductivemembers 1c and 2c and diverge therefrom so as to have the maximumspacing d at the middle portions of the conductive members. It will beapparent that the conductive members 10 and 2c of FIG. 7 are relativelylighter than the conductive members 1b and 2b of FIG. 3, and furtherhave the advantages of a more attractive appearance and a lowerproduction cost.

In suitable dimensional examples of the embodiment of the inventionillustrated by FIG. 7, the wirelike elements 11a, 11b and 12a, 12b havediameters of 6 mm., the loop diameter, that is, the diarnetricaldistance between the middle portions of members 1c and 2c, is 300 mm.,and the maximum width d at W=Wf W" for the lgop antennas having d 50 mm.and

00 mm., respectively. Of course, in the embodiment of FIG. 7, thecharacteristics of which are shown by FIGS. 9A, 9B and 10, theconductive members 10 and 20 have widths that decrease from the maximumvalues d at the middle portions to minimum values at the opposite ends.In order to appreciate the differences between the characteristics ofsuch loop antennas according to this invention, and of similar loopantennas, but in which the widths of the arcuate conductive members arenot thus varied, reference may be had to FIGS. l1, l2 and 13. Thesuitably labeled curves of FIGS. 11, 12 and 13 all refer to loopantennas in which the conductive members are each formed of eithersingle wirelike elements (d=0) or of two parallel wirelike elementsspaced apart by the uniform distances d=50 mm. or d=l00mm. and joined attheir ends by wirelike elements extending therebetween. FIG. 11 showsthe reactance components X, and X a, of the dipole impedance and loopimpedance for the various values of uniform 11; FIG. 12 shows theresistance components R and R for the various values of uniform d which,in this case, affect the values of the resistance components because ofthe varying lengths of the wirelike elements which join the ends of theparallel elements; and FIG. 13 shows the real and imaginary parts W andW" of the image impedances for the various values of uniform W.

Referring now to FIG. 8, it will be seen that, in another embodiment ofthis invention, the maximum width d at the middle of the conductivemembers 1d and 2d constituted by wirelike elements 11a, 11b and 12a, 12bmay be reduced by providing auxiliary conductors 14 and 15 in the formof U- shaped, sheet metal elements joined at their ends to the middleportions of wirelike elements 11a and 11b and to the middle portions ofwirelike elements 120 and 12b, respectively, and directed inwardly fromsuch wirelike elements toward the center of the loop defined thereby. OnFIG. 14, there are shown the real and imaginary part W and W" of theimage impedance of a loop antenna in accordance with the embodiment ofFIG. 8, and in which the dimensions C of the auxiliary conductors 14 and15, that is, the dimensions along the respective members 1d and 2d, areeither 48 mm. or 90 mm.

As will be apparent from FIG. 14, loop antennas of the type shown byFIG. 8 are particularly suited for use in connection with televisionreceivers, because such antennas have image impedances that aresubstantially the same for all standard television channels.

Referring now to FIGS. 15 and 16, it will be seen that, in a practicalembodiment of this invention particularly suited for use with portabletelevision receivers, the loop antenna is constituted by a pair ofconductive members 10 and 2c of the type described above in connectionwith FIG. 7, and which have their adjacent ends joined to each other byway of an elongated, insulated holder 16 which extends diametricallyacross the loop. The holder 16 is mounted, at its middle, by way of auniversal joint 17, on the upper end of a rod 18 which, at its lowerend, is rotatably mounted, as at 19, on a base 20 by which the loopantenna may be supported on vart alak grothe horizontal surface. Byreason of the universaljoint 17 and rotatable mounting of rod 18, theloop antenna constituted by conductive members 10 and 2c is rotatableabout three orthogonally related axes so as to be positionable for bestreceiving television waves arriving in any direction.

As shown on FIG. 17, when the arriving wave A is generally in the planeof the antenna loop, a cardioid directivity characteristic 21 isobtained. However, when the arriving wave A is at an acute angle to theplane of the antenna loop, the directivity pattern is a shown at 22 onFIG. 18, and a figureeight directivity characteristic 23 results whenthe arriving wave A is substantially perpendicular to the plane of theantenna loop, as shown on FIG. 19.

Although illustrative embodiments of this invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments and that various changes and modifications may be effectedtherein by one skilled in the art without departing from the scope orspirit of the invention.

We claim:

1. A loop antenna comprising a pair of conductive members, each of saidconductive members being arcuate and including a pair of elongatedarcuate wirelike elements joined together at the opposite end portionsof the respective conductive member and diverging from said end portionsto the middle portion of the respective conductive member to vary thewidth of the latter in directions that are perpendicular to thedirection of curvature thereof, means mounting said conductive membersin opposed relationship with their concave sides facing toward eachother to cooperate in defining a loop, each of said conductive membersfurther including a generally U-shaped, sheet metal element having itsends joined to said wirelike elements of the respective arcuateconductive member adjacent said middle portion of the latter anddirected inwardly from said wirelike elements toward the center of saidloop, a dummy load connected between one end of one of said conductivemembers and the adjacent one end of the other of said members, andoutput means connected to the other ends of said conductive members.

1. A loop antenna comprising a pair of conductive members, each of saidconductive members being arcuate and including a pair of elongatedarcuate wirelike elements joined together at the opposite end portionsof the respective conductive member and diverging from said end portionsto the middle portion of the respective conductive member to vary thewidth of the latter in directions that are perpendicular to thedirection of curvature thereof, means mounting said conductive membersin opposed relationship with their concave sides facing toward eachother to cooperate in defining a loop, each of said conductive membersfurther including a generally U-shaped, sheet metal element having itsends joined to said wirelike elements of the respective arcuateconductive member adjacent said middle portion of the latter anddirected inwardly from said wirelike elements toward the center of saidloop, a dummy load connected between one end of one of said conductivemembers and the adjacent one end of the other of said members, andoutput means connected to the other ends of said conductive members.